Add odds ratio helpers

ratio.py

@@ -0,0 +1,6 @@
+def odds_ratio(p1, p2):
+    """Odds ratio between two chances."""
+    return (p1 / (1 - p1)) / (p2 / (1 - p2))
+
+def margin(prices):
+    return sum(1.0 / p for p in prices) - 1.0